An Investigation of a Measure

The equal environments assumption, which holdsthat trait-relevant environments are equally corre-lated among monozygotic (MZ) and dizygotic (DZ) twin pairs, is essential to twin designs. Violations of this assumption could lead to biased parameter esti-mates in twin models. A variety of methods and measures have been used to test this assumption. No studies to date have evaluated the measurement invariance of such items or examined the distribu-tion of the underlying equal environments trait. The current study was an investigation of the psychome-tric properties of a self-report measure of twins’ equal environments. Exploratory and confirmatory factor analysis results indicated that items loaded onto ‘child ’ and ’teen ’ equal environments factors. Factor loadings and factor variances and their covari-ance were invariant for MZ and DZ twins; however, DZ twins had significantly lower factor means than MZ twins. Further, these items demonstrated ade-quate test–retest reliability. Lastly, the child and teen factors may be bimodally distributed, particularly for MZ twin pairs. Measurement invariance issues, as well as distributions of equal environments traits, should be considered when evaluating the equal environments assumption, in order to produce accu-rate parameter estimates in twin models